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1
Preheat oven to 400 F. Butter a 9x13-inch baking dish and set aside. Add the pasta to a large pot of boiling salted water and cook the pasta until it is al dente. Drain the pasta in a colander then return to the pot.
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2
While the pasta is cooking, melt 2 tablespoons of the butter in a large skillet over medium-high heat. Add the mushrooms and shallots to the pan and saute, stirring occasionally, until the mushrooms are tender and most of the liquid has evaporated from the pan, about 6-8 minutes. Add the garlic and cook for about 30 seconds more, just until the garlic is fragrant. Remove the pan from the heat and season the mushroom mixture with salt and pepper to taste. Mix in the vinegar and then set the pan aside.
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3
To make the sauce, melt 4 tablespoons of butter in a medium saucepan over medium-high heat. Once the butter has melted, add the flour and cook, whisking constantly, until the mixture is golden, about 1-2 minutes. Stir in the herbs and cook just until fragrant, about 30 seconds. Add the milk to the pan and cook, whisking constantly, until the sauce comes to a simmer and thickens. Once thickened, remove from the heat and add the shredded Gruyere and cheddar cheeses, whisking until they are completely melted and the sauce is smooth.
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4
Pour the sauce over the cooked pasta and stir to coat. Add the mushroom mixture and mix to combine. Transfer to the prepared baking pan.
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5
Melt the remaining 2 tablespoons of butter. Pour the butter over the breadcrumbs and use a fork to coat the crumbs evenly. Sprinkle the breadcrumbs evenly over the top of the pasta.
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6
Bake for 18-20 minutes, until browned and bubbly. Let stand for 10 minutes before serving.
